From the point of view of energy saving and environmental protection, a lightweight technology for various transportation means, such as automobiles, aircrafts, railway vehicles and ships, has emerged. In a welding process of lightweight materials, a friction stir welding (FSW) technology which is a non-melting solid state welding technology is applied. Recently, the FSW technology spotlighted as a next generation technology is widely used in welding of similar and dissimilar materials of high melting point materials, such as titanium, steel, stainless and nickel alloys, as well as the lightweight materials, and also applied to various industrial fields.
In order to weld the high melting point materials, it is necessary to develop a tool material having a long lifespan, and thus various materials are developed and studied to satisfy high strength, high abrasion resistance, high toughness, uniformity of micro-structures and the like. For example, as this kind of material, there are PCBN prepared by MegaStir company located in U.S.A., and Ir—W, Ir—Re, Ir—Mo or the like prepared by Furuya company located in Japan. However, the tools prepared in U.S.A. and Japan have excellent strength and toughness, but have disadvantages of a high price and a short lifespan.
Tungsten carbide (WC) has a melting point of 2600° C., and a density of 15.7 g/cm3, and cobalt (Co) has a melting point of 1459° C., and a density of 8.9 g/cm3. WC—Co is called as cemented carbide, has advantages in ceramics and metals and thus is applicable for various purposes. Since the tungsten carbide has a high melting point, high strength and high abrasion resistance, it is used for various purposes, such as machining tools, abrasion resistive tools, cutting tools and molds, and when the Co is added, toughness is improved, and thus it is possible to prepare a material having high toughness.
Recently, a preparation method of the WC—Co which is spotlighted as a material for a solid state friction stir welding tool can be classified into a melting/casting method, and a powder metallurgy method. The melting/casting method is the most general methods of sintering and preparing the WC—Co, which have some advantages of facilitating mass production and reducing preparation costs, but also have some disadvantages of limiting control of fine particles and high density. Furthermore, the above methods require several post-treatment processes.
However, in case of using the powder metallurgy method, since it has advantages of uniform phase distribution, control of fine particles and facile preparation of a high melting point material, and also has an advantage which can prepares the WC—Co having the high toughness and high strength, it is widely used as a substitute process for the melting/casting method.
However, as conventional powder metallurgy methods, a hot isostatic processing (HIP) method and a hot pressing (HP) method which can obtain a sintered body having a relative high density by simultaneously applying temperature and pressure have been mainly used, but development of a new process technology is required due to deterioration of strength, toughness and abrasion resistance caused by a long molding process time and thus limitation of fine particle control, and a difference between internal and external physical properties and an expensive process cost caused by an external heating system.
To satisfy the requirement, the applicant has filed a patent application for a preparation method of a WC—Co sintered body, which can be used for the solid state friction stir welding tool, using a discharge plasma sintering process.
However, in the WC—Co sintered body, since it is necessary to add Co to WC, additional processes are required over multiple stages in the preparation method. When preparing the sintered body, a process cost is increased and hardness thereof is reduced due to the addition of the Co, and thus it is restricted to be used as the high melting point friction stir welding tool due to a short lifespan. Therefore, a new method which can prepare a sintered body for the solid state friction stir welding tool using only the WC without a sintering additive such as Co is required.